double email or no email

We acquired Webburger recently and are now supporting their users. We are trying ot get them to move to our new andHosting.com servers, but until they're all moved we have to support the old Webburger servers as well.

On one of these older non-cpanel servers, one of our customer domains is having email problems. Two of the addresses don't get any email, and two others get duble email (two of everything). I looked through the logs as best as I could, but didn't see anything. How can I diagnose something like this?